Communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
With an increasing demand from the network society, either on huge traffic volume or on very low latency, communications network need to be continuously evolved to meet such demands. Some consensus have been reached about how to meet the demand, and two methods are considered to be very important. The first method is to densify the communications network, and the second method is to use more frequency spectrum. Due to an insufficient frequency spectrum, the available frequency for next generation communications network would probably be at very high frequencies, e.g. from 10 GHz and above.
However, for such high frequency spectrums, the atmospheric penetration and diffraction attenuation is much worse than for low frequency spectrums. More importantly, the receiver antenna aperture, as a metric describing the effective receiver antenna area that collects the electromagnetic energy from an incoming electromagnetic wave, is frequency dependent, i.e., the link budget would be worse for the same link distance even in free space scenario, if omni-directional receive and transmit antennas are used. This motivates the usage of beam-forming in order to compensate for the loss of link budget in high frequency spectrums. Beam-forming may be used at the transmitter, at the receiver, or both. In a large part of the spectrum planned for 5G deployments, the preferred configuration is to use a large antenna array at the Access Node (AN) and a small number of antennas at the wireless device. In the DL, this means that high-order transmit beamforming may be applied by the AN transmitter in the DL.
In current LTE systems, a wireless device sends a Physical Random Access Channel (PRACH) preamble sequence to one or more Radio Network Nodes (RNN), e.g. eNodeBs, based on broadcasted system information and on random access configuration. The timing of each PRACH transmission from the wireless device is based on one or more received downlink Primary Synchronization Signals (PSS) and/or Secondary Synchronization Signals (SSS) from one or more RNNs. In the PRACH preamble the Guard Time (GT) field is designed to cover propagation delay, so that the preamble may be detected by the RNN, assuming a zero timing advance in the uplink subframe. FIG. 1A schematically illustrates time slots for a PRACH preamble for a first wireless device close to the eNodeB and for a second wireless device at the cell edge, respectively. As illustrated, the PRACH preamble comprises a Cyclic Prefix (CP) and a sequence, sometimes herein referred to as a PRACH preamble sequence. Both the first and the second wireless devices align the start of the PRACH preamble with the start of the corresponding uplink subframe at the wireless device assuming a Timing Advance (TA) of zero. The two preambles are received at the eNodeB with different timings depending on the propagation delay in the uplink. The GT is designed to the cover the max Round Trip Delay (RTD) so that the preamble from the second wireless device, i.e. the cell edge UE, is also received in the PRACH time slot.
FIG. 1B schematically illustrates the PRACH timing according to prior art LTE systems. As illustrated in FIG. 1B, and since it is not synchronized among different ANs, the wireless device needs to send three preambles to the three ANs at different points in time if the wireless device requires the three ANs to receive the preamble. In other words, the wireless device needs to send a first preamble at a first point in time if the preamble should be received by the first AN AN A, a second preamble to the second AN AN B at a second point in time if it should be received by the second AN AN B, and a third preamble to the third AN AN C at a third point in time if it should be received by the third AN AN C. The points in time depends on the downlink sync signals from the different ANs. In FIG. 1B, the first preamble for the first AN AN A is sent at the first point of time t_AN_A, the second preamble for the second AN AN B is sent in at the second point of time t_AN_B, and the third preamble for the third AN AN C is sent in at the third point of time t_AN_C. As illustrated in FIG. 1B, the time difference between the second point in time and the first point in time is given by Δt_AN_B, and the time difference between the third point in time and the first point in time is given by Δt_AN_C.
A drawback with the prior art is that the time to send the PRACH preamble sequence to the eNodeB is based on the broadcasted system information and random access configuration from one eNodeB, e.g. a single eNodeB. This is especially a drawback when it is difficult for several eNodeBs to simultaneously receive the PRACH and when the several eNodeBs are not perfectly synchronous with each other.